Extended copy and error message "Interner Fehler ... Objektverweis ... Objektinstanz"; substantial increase of data size of project

I receive the following error message when trying to copy a device with the function "extended copy":

"Ein interner Fehler ist aufgetreten. Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt."

The new device will not be created.

Also the size of exported project has increased from 6.907KB to 10.072KB. Main changes I did between the two exports were importin a product catalog (Arcus EDS), deleting some group adresses and adding a new device from Arcus EDS.

Can you help?

Thanks in advance.